Regulators, or converters, including a switch, sometimes referred to as a power switch, for transferring energy from an input, such as an AC or DC voltage or current source, to a regulated output are well known. In some regulators, sometimes referred to as switching regulators, the switch turns on and off to regulate the output. Such switching regulators include an inductor to convert the switched current pulses into a continuous load current. In other regulators, sometimes referred to as linear regulators, the switch operates in its active, or saturation region.
An important specification of regulators is the desired average output level, which may be referred to as the regulation point. The regulation point is generally specified to have a permissible range to which the regulator will maintain the output over specified conditions, such as load and temperature conditions, as well as over process and circuit variations, such as offsets and gain errors.
Common switching regulator configurations include Buck, Boost, Buck-Boost, flyback, SEPIC, Cúk, half bridge, and full bridge to name a few. As is also well known, various control methodologies for controlling conduction of the power switch can be applied to switching regulators, including Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM), and for each of these methodologies, various control techniques are possible including voltage mode control and current mode control along with various feedback and feed forward techniques.
The different methodologies for controlling conduction of the power switch have different advantages and disadvantages. For example, in the case of PWM control, the switch is operated at a fixed frequency, but at a variable duty cycle in order to regulate the output. Since the variable duty cycle permits the peak inductor current to be adjusted as necessary to maintain the desired output level, PWM control is conducive to a wide range of load levels. However, at light loads, PWM control can result in undesirable power dissipation and therefore reduced efficiency since the switch operates at a fixed frequency regardless of the load requirements. In the case of PFM control on the other hand, the switch control signal has a fixed on time, off time, or peak inductor current, but a variable frequency in order to regulate the output. Under PFM control, the lower switching frequency reduces the efficiency loss that results from turning on and off the power switch, but the peak inductor current is limited and thus, PFM control is conducive to lighter load conditions where the limited peak inductor current is sufficient to supply the load.
Some conventional switching regulators can operate in two or more regulation modes in order to get the benefits of different switch control methodologies. For example, in a U.S. Pat. No. 5,481,178 entitled “Control Circuit and Method for Maintaining High Efficiency over Broad Current Ranges in a Switching Regulator Circuit,” a current comparator controls switching during higher load current conditions and a hysteretic comparator puts the regulator into a sleep mode during lighter load current conditions. In regulators having two or more regulation modes, the operating conditions under which the regulator transitions between modes can be difficult to establish without adversely impacting efficiency.